1. Field
This invention relates to a welding process, in particular a shielded gas tungsten arc welding (GTAW) process.
2. Related Art
An improved welding process for butt welding steel workpieces along bevel prepared joints using GTAW equipment with filler wire and hydrogen-argon (or other inert gas) shield gas mixtures during the first or root pass and then hydrogen-free inert gas during the subsequent filler passes is disclosed in U.S. Pat. No. 5,686,002 issued Nov. 11, 1997, this patent owned in common with the invention described and claimed herein.
In accordance with the process described in U.S. Pat. No. 5,686,002, medium to thick sheet or tubular steel metal workpieces are prepared by beveling their edges to be joined so they butt against each other with a small clearance (“open root”). A root weld pass is then made using conventional automatic (including semi-automatic) or non-automated GTAW equipment, filler wire and a shield gas composed preferably of a 95/5 argon-hydrogen mixture.
Successful welding of such workpieces, in particular workpieces made of carbon steel, was originally doubtful due to constraints against allowing hydrogen into carbon steel weldments because of embrittlement and porosity problems encountered in prior art welding procedures used with such metals when hydrogen contamination occurred.
On the other hand, it was theorized that the presence of hydrogen during the root pass would solve penetration, weld pool stability, cleanliness and other problems faced by welders joining carbon steel workpieces such as large diameter tubes and pipes that were prepared using beveled edges with open roots because of the enhanced heating effect obtained with hydrogen in the shield gas mixture.
The solution as described in U.S. Pat. No. 5,686,002 was to use the argon/hydrogen mixture only for the root pass and then use a hydrogen-free inert gas mixture for the subsequent passes, which theoretically would purge any free hydrogen from the weldment and produce a sound, non-porous weld that would not be subject to hydrogen embrittlement. Indeed, testing involving carbon steel as well as high strength steel such as chrome-molybdenum steel has shown that such welds that are free of adverse effects of hydrogen in the shield gas can be obtained using this novel procedure. Reference may be made to U.S. Pat. No. 5,686,002, which is incorporated herein in its entirety by reference, for a more detailed description of the welding process described therein.
While the welds produced using the method described in U.S. Pat. No. 5,686,002 are sound and of good quality, an unexpected phenomenon was observed when implementing the process to weld carbon steels that initially defied explanation.
Specifically, when the second weld pass was made over the root pass, sudden virtually explosive eruptions of fused metal in the weld pool area occurred with sufficient frequency and intensity so as to interrupt the welding process by contaminating the electrode and gas lens of the GTAW torch equipment. The eruptions were observed to actually deposit weld metal on the electrodes and gas lens to the extent that they required repair or replacement before welding could continue. This phenomenon was observed even during a second pass “dry wash” (fusion welding without filler) was being made.
An explanation was not readily at hand, as the root pass welds and the cover or filler welds were made strictly in accordance with all acceptable GTAW standards and specifications, including avoiding contamination of any kind in the weld zone during welding, in particular moisture and oxygen contamination. All the equipment of the GTAW apparatus was examined as was the shield gas supply. Electrodes were suspect initially, but that proved to be unproductive as a causation factor. Possible explosive chemical contamination was quickly ruled out, as was a possible combination of elements produced during welding that could result in the presence of a potentially explosive material or gas in the solidified weld metal of the root pass. Varying the mixture of the filler weld shield gas did not indicate that the shield gas as supplied was a source of the problem either. Moisture in the weld zone also was ruled out by experimentation involving preheating the workpieces.
It was eventually theorized that the expulsive events involving fused weld metal during the first filler or cover pass could be resulting from superheated gas of some kind possibly trapped in small pockets or cells in the solidified weld metal of the root pass that, as the leading edge of the pool approaches the pocket or cell and heats the area of metal surrounding the pocket or cell, is intensely pressurized until, when fusion of the surrounding metal occurs, the superheated and pressurized gas violently expands in an intense expulsion of fused weld metal. It was theorized that the use of hydrogen during the root pass could lead to some porosity initially that would be removed during the next cover or filler weld pass. Yet, the presence of small pockets of mostly argon and minor quantities of free hydrogen hardly would account for the violent nature of the eruptions that were observed during the first filler pass over the root weldment.
The mystery remained unsolved until further research pointed to the possibility that somehow trapped moisture could be contained in the cells or pockets of the root pass weld. How the errant moisture came to be lodged in the cells or pockets could not be envisioned upon initial investigation, as all precautions were taken to avoid moisture contamination in the shield gas and in the weld zone. If trapped moisture was contained in the cells, on the other hand, at least this could account for how the trapped vapor in the cells, being superheated into steam during the second pass, could cause the eruptions that were observed.
But how the moisture reached the weld area remained the question to be answered.
U.S. Pat. No. 2,905,805 issued in 1959 reviewed by the inventor during research concerning the weld metal expulsion problem revealed difficulties encountered with moisture pockets created during GTAW welding of stainless steel using an argon-hydrogen shield gas mixture comprising 35% hydrogen. In accordance with this patent, the chemical combination of hydrogen and oxygen during welding was believed to create moisture that lodged in pores of the weldment and the solution proposed in the patent was to limit the amount of hydrogen present at the weld zone in the shield gas and adjust the speed of weld tool advancement.
This theory did not seem plausible in the context of carbon, chrome-molybdenum, and other non-stainless steel welding with GTAW equipment and a 95/5 argon-hydrogen shield gas mixture. In the first place, no oxygen contamination in the weld zone was believed to be present due to the very nature of the shield gas system that maintained the weld zone oxygen-free at all time. There simply was insufficient oxygen to combine with the little hydrogen in the shield gas that was not consumed during welding to create moisture in the weld metal, irrespective of possible occurrence of cells or pockets in the weld metal due to other causes. Carbon steel welds were known to be subject to porosity to a considerably greater extent than stainless steel welds, but how moisture could be present in the cells of the pores of the carbon steel weld made in accordance with the process of U.S. Pat. No. 5,686,002 was not readily explainable.
Attention then focused on the shield gas delivery system as a possible source of moisture and possibly oxygen. It was theorized that perhaps the very substance that was intended to maintain the weld zone free of moisture and oxygen contamination, namely the shield gas, was itself transporting the harmful contamination despite all precautions.
The shield gas storage tanks, the fittings, the delivery hoses and the welding heads used for the supply and delivery of the shield gas mixture were all examined in detail for possible sources of contaminating moisture and oxygen. Still the source of such elements eluded discovery.
The elastomer shield gas delivery hoses then were examined as possible sources of hydrogen and oxygen contaminating the weld zone during the GTAW welding procedures carried out in accordance with U.S. Pat. No. 5,686,002. Although the hoses in use met all specifications for GTAW welding, it was learned that various plastic, rubber and other elastomer hose materials available for welding gas supply applications are subject to various degrees of gas permeation or diffusion through the wall of the hoses, depending on the material used to construct the hoses and other factors. Experimentation then resulted in the discovery that the likely source of the moisture in the cell pockets of the welds that were reacting in the manner described above was the elastomer shield gas delivery hoses that were permitting moisture vapor from atmosphere to permeate or diffuse through the hose walls to a sufficient degree to become entrapped in the cells or pockets of the root pass weld. Upon heating during the subsequent weld pass, the moisture was superheating and creating the violent expulsion effects observed. A solution to the weld metal expulsion events was needed to avoid the disadvantages that resulted from such expulsion.
As a secondary factor, the tungsten electrodes conventionally used in GTAW equipment were observed to degrade severely during use in the welding process according to U.S. Pat. No. 5,686,002, that is using 95/5 argon-hydrogen gas mixture. Unlike stainless steel welding procedures using 95/5 argon-hydrogen shield gas where electrode life was normal, or “clean room” environment welds involving high shielding environments, the process according to the said patent involved welding in environments not necessarily intended to produce “clean” or sterile welds. This typically is of little concern where stainless steel “clean room” welding is carried out, as such welding uses relatively low amperage and heat input and welds are of shorter duration than typical welding procedures involving, for example, carbon steel plates and heavy wall tubes, or other steels such as chrome-molybdenum. In a “clean room” processes, the doping agents in the tungsten electrodes used may not vaporize completely, thereby possibly reducing oxidation and condensation of oxides onto the electrode during use. On the other hand, tungsten electrodes used during a welding process according to the U.S. Pat. No. 5,686,002 patent with carbon, chrome-molybdenum and other non-stainless steels exhibited or will be expected to exhibit shorter life and to require replacing at frequent intervals, possibly due to longer welding times, higher heat input due to the hydrogen in the shield gas, oxide condensation on the electrodes, weld zone contamination at the location of the electrodes and other unidentified causes.
Clearly a solution to short tungsten electrode life in a welding process involving carbon steel-in particular, but any non-stainless steel generally using a partial hydrogen shield gas was needed.